A PDP (plasma display panel) is roughly classified into an AC type PDP and a DC type PDP from the characteristics of its electrode structure.
As shown in FIG. 3B, the AC type PDP has a structure in which the surface of an electrode 2 is covered with a dielectric layer 3 in which an electrostatic capacitance 7 is formed, the surface of the dielectric layer being covered with a dielectric material 5 such as magnesium oxide with a high secondary electron radiation property. On the other hand, the DC type PDP is characterized by a structure in which the surface of an electrode is not covered with a dielectric layer but exposed to the discharge space to directly radiate secondary electrons from the surface of the electrode although not shown.
Since the ordinary AC type PDP has a so-called reflection type structure in which a discharge electrode is disposed on the front surface side, the electrode 2 should be formed as a transparent electrode. In general, an indium tin oxide layer, which might be called an ITO layer, is high in electric resistance and hence the resistance should be lowered by compensating for the electric resistance. Thus, it is customary that a metal electrode with a high conductivity, which might be called a bus electrode 9, is superposed upon the electrode 2.
From an operation standpoint, the above two plasma display panels have the following characteristics. The AC type PDP is characterized in that charged particles generated by discharge are accumulated on the surface of the dielectric layer covering the electrode 2 and the surface of the magnesium oxide layer 5 to form so-called wall electric charges, charges being continued with application of an AC type pulse voltage to the place between a pair of the electrode 2 and the bus electrode 9 by using a so-called wall voltage produced therein to render the whole of pixels memory functions. Since the DC type PDP is not given the above-described memory function because the surface of the pixel is conductive but it is characterized in that a discharge current of a direct current continues to flow during a time period in which it is being applied with a constant discharge current to thereby discharge to emit light.
As described above, although the AC type PDP is featured in that electric charges are accumulated on the surface of the electrode, since a material of the dielectric layer formed for that purpose, that is, a low melting-point glass is low in secondary electron radiation rate and has poor durability against ion bombardment, the surface of this dielectric layer should be coated with a material such as the above-mentioned magnesium oxide MgO having a high secondary electron radiation rate and which is strong against ion bombardment as the protective layer of the cathode layer and the dielectric layer.
In this case, in order to enable the electrode 2 with the above-mentioned structure to operate as the AC type electrode, this protective layer 5 should be made of a dielectric material to accumulate wall electric charges on the surface of the cathode layer and protective layer 5.
Also, in addition to the AC type PDP having the fundamental structure shown in FIG. 3B, there has been proposed an AC type PDP having a structure whose structure and operation are the same as those of the AC type PDP with the fundamental structure but in which pad-like intermediate layers 8 are laminated on the pair of opposing discharge electrodes 2 at their distant portions through dielectric layers as is shown in a cross-sectional view of FIG. 3C. Also in this case, since the pad-like intermediate electrode 8 is covered with the MgO layer 5, its operation is the same as that of the AC type PDP with the fundamental structure.
As described above, in the conventional AC type PDP, since the surface of the dielectric layer should be covered with other dielectric layer serving as the cathode layer and protective layer, its material has to be selected in an extremely narrow range and only the magnesium oxide MgO is used as such material in actual practice.
However, such oxide material is very unstable from a property standpoint and hence it is difficult to make. Although it is customary to form such oxide material by a vacuum deposition method or a sputtering method, any one of methods needs a long treatment time because the whole of substrate is treated by a heating treatment within a vacuum apparatus which is highly evacuated.
Further, the manufacturing process has encountered with a serious problem in which MgO is high in hygroscopic property so that it is easily changed into Mg (OH)2, that is, magnesium hydroxide, its function as the cathode material being lost. Hence, its process has been regarded as the most difficult process in the manufacturing process of PDP.